1. Field of the Invention
The present invention relates to a toner for use in electrophotography and in toner jetting and image forming methods for visualizing electrostatic images.
2. Description of the Related Art
The requirements of higher speeds and higher reliably placed on electrophotographic image-forming apparatuses have become more demanding in recent years. Requirements concerning, for instance, power saving and shorter wait times have become likewise more demanding. To meet these demands, toners are required to afford low-temperature fixability in high-speed developing systems.
Fixing performance is ordinarily correlated with toner viscosity, but in high-speed developing systems, in particular, the conventional requirement of fixing performance is compounded with the need for quick melting with a small amount of heat during fixing (so-called sharp melt property).
Japanese Patent Application Publication No. 2007-58135 proposes a toner having improved low-temperature fixability, obtained by bonding, to an amorphous polyester resin, at least one monovalent aliphatic compound selected from the group consisting of monovalent aliphatic carboxylic acid compounds having 10 to 24 carbon atoms and monovalent aliphatic alcohols having 10 to 24 carbon atoms.
When bonded to the amorphous polyester resin, the resulting aliphatic hydrocarbon segment plasticizes the resin, whereby low-temperature fixability is enhanced.
When the toner is stored at high temperature, however, the amorphous polyester segments are gradually plasticized by the aliphatic hydrocarbon segments of high molecular mobility, and heat-resistant storability is impaired as a result. Further, the difference in viscosity between the portions plasticized by the aliphatic hydrocarbon segments and other portions of the amorphous polyester resin increases during hot melting, and gloss unevenness may consequently arise in fixed images. Although the above feature is effective as regards low-temperature fixability, there is thus still significant room from improvement in terms of heat-resistant storability and other properties.
There are numerous reports (for example, Japanese Patent Application Publication No. 2003-337443) on the use of a binder in the form of a crystalline resin instead of an amorphous resin, with a view to imparting a sharp melt property.
As is known, crystalline resins melt rapidly, at about the glass transition temperature, and thus low-temperature fixability can be improved on account of higher compatibility with the amorphous resin.
If the compatibility between the crystalline resin and the amorphous resin is excessively high, however, the heat-resistant storability of the toner becomes poorer and the sharp melt property of the crystalline resin is lost, as a result of which the fixing performance may be impaired in the high-speed developing system.
Accordingly, toners have been proposed (Japanese Patent Application Publications No. 2010-107673 and 2008-203779) which, in terms of controlling compatibility, rely on a combination of a crystalline polyester resin and an amorphous polyester resin having bonded thereto an aliphatic hydrocarbon segment of a certain number of carbon atoms. It has been suggested that a toner having superior fixing performance, storage stability, developing characteristics and so forth can be achieved by virtue of that feature.
Although a certain effect on fixing performance is found to be elicited in all the above instances, it is difficult to reliably avoid a state where the amorphous polyester resin is readily plasticized by the aliphatic hydrocarbon segment that is bonded to the latter. In particular, the heat-resistant storability of the toner may decrease when the toner is left to stand at high temperature over long periods of time.
Thus, no toner has been provided thus far that is sufficiently satisfactory as regards fixing performance during high-speed development, long-term storage stability, high-temperature high-humidity storage stability, and, in addition, gloss uniformity.